The hunt for an effective, reversible, and socially acceptable male birth control continues. The newest target: The smooth muscle that makes up the tubes connecting the testes to the urethra. This needs to contract in order for sperm to reach their final destination. Now, scientists have shown that you can make mice sterile by eliminating their ability to contract that muscle. The result: A mouse with a dry ejaculation but which is still "pelvis thrusting with appropriate vigor and frequency".

This is a long way from becoming reversible treatment for human gentlemen, though. Right now, probably the most promising male birth control is RISUG, in which a clear polymer gel is injected into the vas deferens. The gel doesn't block the tube up completely, but it does seem to prevent sperm from successfully reaching the urethra and being capable of fertilizing an egg. RISUG is in Phase III clinical trials in India, but, even then, there are still safety questions about it and, so far, it's only been proven to be reversible in tests on non-human primates.

IUDs are the weird form of birth control. We don't really know exactly how they work, for instance. And they've been largely unpopular my entire lifetime—really, ever since a couple of poorly designed IUDs set off a mini-panic in the late 1970s and early 1980s. But IUDs are effective birth control. The ones that you can buy today are safe. And, more importantly, they represent birth control that you don't have to think about, and birth control that is really hard to get wrong.

If you've ever done research on the effectiveness of various methods of birth control, you'll notice that the statistics usually come with a little asterisk. That * represents a concept that few of the people who rely on birth control ever think about—perfect use. Let's use condoms as an example. With perfect use, 2 out of 100 women will get pregnant over the course of a year's worth of condom-protected sex. Without perfect use—maybe you don't use a condom every time, maybe you don't put it on right when you both get naked—the number of accidental pregnancies jumps to 18 out of 100. The same basic problem affects birth control pills, as well. Ladies, did you know you're supposed to take those things at the same time of day every day? That's the kind of use error that can make a difference between 1 out of 100 women getting pregnant in a year, and 9 out of 100 getting pregnant.

In contrast, IUDs represent a fit-it-and-forget-it method of birth control. Which is a big part about why they're up there with outright sterilization as the most effective means of birth control available. Bonus: Depending on which kind you use, you can avoid hormonal side effects. This, experts say, is why IUDs are experiencing something of a resurgence in popularity. In an article at Wired, Jennifer Couzin-Frankel writes that 5.5 percent of American women who use birth control use IUDs. That's up from only 1.3 percent in 1995.

Somewhat unbelievably, no one is quite sure how they work, but the theory goes like this: The human uterus has one overriding purpose, which is to protect and sustain a fetus for nine months. If you stick a poker-chip-sized bit of plastic in there, the body reacts the way it does to any foreign object, releasing white blood cells to chase after the invader. Once those white blood cells are set free in the uterus, they start killing foreign cells with efficient zeal. And sperm, it turns out, are very, very foreign. White blood cells scavenge them mercilessly, preventing pregnancy. In copper- containing IUDs, metal ions dissolving from the device add another layer of spermicidal action.

... Most modern IUDs incorporate copper, which has an assortment of benefits, including increased durability and effectiveness. They’re also free of hormones and can be made cheaply, a boon for women in developing countries. But copper IUDs can cause heavy menstrual bleeding and cramping. The Mirena solves that problem by forgoing the metal for a synthetic version of the hormone progesterone. Here again, the mode of action isn’t completely understood, but researchers suspect that the hormone thickens cervical mucus, which makes it nearly impossible for sperm to swim upstream. It may also thin the uterine lining, rendering it inhospitable to an embryo should fertilization occur. The hormone-based IUD has the opposite side effect of the copper ones: It sometimes leaves women with little uterine lining to shed, so they hardly get any period at all.

... Even though many more doctors are comfortable with the IUD, a generation of doctors didn’t get practice inserting it. And if they don’t know how to put one in, they’re less likely to recommend it as an option. Also, the devices are expensive—the ParaGard costs $500, the Mirena $850. “It’s absolute highway robbery that these companies charge so much,” Espey says. “If you went to Home Depot and got the raw materials for a copper IUD, it would cost less than 5 cents.” And the hormones don’t contribute much more to the cost, she adds. In fact, amortized over years of use—10 for the ParaGard and five for the Mirena—an IUD is far cheaper than birth control pills, which can cost $30 or more a month. But the initial outlay is difficult for some women to manage, and it’s not always covered by insurance.

Really interesting new study of 20,000 women suggests that the use of IUDs might reduce the risk of both major types of cervical cancer, even in women who contracted cancer-causing HPV. The researchers speculate that the IUD's presence—it is, after all, a foreign object in your lady bits—may serve to stimulate immune responses that fight off HPV infection early and prevent it from progressing to cancer. This needs follow up. But it's intriguing. (Via Colleen McCaffery)